1. Field of the Invention
This invention relates to an ink used in an image forming process in which ink droplets are ejected from a recording head by the action of thermal energy to record images on a recording medium, and to an ink-jet recording apparatus making use of such an ink. More particularly, it relates to an ink-jet recording pigment ink having been improved dramatically in ejection stability required when a pigment ink achievable of a high fastness property of images is ejected from the recording head of the above system, and to an ink-jet recording apparatus making use of such an ink.
2. Description of the Related Art
Various systems have conventionally been proposed in the printing that employs an ink-jet recording process. For example, Japanese Patent Application Laid-open No. S54-051837 discloses an ink-jet recording process in which ink is ejected in the form of ink droplets by the action of thermal energy [what is called a BUBBLE JET (registered trademark of CANON INC.) process]. In particular, this recording process facilitates a high-density multi-nozzle very much, and hence has characteristic features that high-quality images are obtainable at a high speed and at a very low cost, and that images are printable also on plain paper, which has any special coating layer. To describe this recording process in detail, first, a heater of the recording head is rapidly heated, whereupon a liquid on a heater comes into air bubbles to cause an abrupt increase in volume. Next, in virtue of impact pressure standing on this abrupt increase in volume, ink droplets are ejected from nozzles at the tip of the recording head and fly therefrom to the recording medium to adhere thereto, thus printing is performed.
Here, in the ink-jet recording process as described above, it is usual to use aqueous dye inks making use of dyes as coloring materials. Recently, however, it is attempted to use aqueous pigment inks making use of pigments as coloring materials. What is given as a reason therefor is that images recorded using the aqueous pigment inks can be superior in fastness property such as water resistance or light-fastness property, compared with images recorded using the aqueous dye inks. On this matter, aqueous pigment inks satisfying basic properties such as image recording quality level, ink ejection performance, storage stability, anti-clogging and fixability which are required in ink-jet recording inks are proposed as disclosed in, e.g., Japanese Patent Applications Laid-open No. H02-255875, No. H04-334870, No. H04-057859 and No. H04-057860.
As stated previously, the BUBBLE JET (registered trademark) process is greatly characterized in its possibility for higher nozzle density, and is an image forming process which is of a superior system among any ink-jet recording processes. However, when an aqueous pigment ink is ejected by such a process as the BUBBLE JET (registered trademark) process, it comes about that the ink is exposed to high temperature at the heater surface of the recording head, and hence a problem as stated below may arise. In the system as above, components contained in the pigment ink may change in properties because of an effect due to the high temperature at the heater surface of the recording head to become accumulated on the heating element surface as deposits. Then, with accumulation of such deposits, it may come about that the heater of the recording head comes to have poorer heat conduction performance with time, so that the impact pressure accompanying the bubbling of ink becomes lower, resulting in deterioration of ejection performance, and furthermore resulting in a poor quality of recorded images. Hence, in order to make pigment inks favorable for the ink-jet recording system in which ink droplets are ejected from a recording head by the action of thermal energy, it is sought to provide an ink that has resolved the above problem, in addition to the above basic properties.